This invention relates to a mould for forming apertures in plastic resin sheet materials.
Divers kinds of carton formed from sheets of plastic resin materials such as foamed polystrene include latching or locking devices which are used to keep the cover or lid of the carton in a closed position. Egg cartons such as those disclosed in U.S. Pat. Nos. 2,990,094 Reifers, 3,648,916 Commisso, 3,687,350 Warburton, 3,735,917 Warburton and 3,908,891 Jackson, are the most typical of this kind of carton although cartons for other articles, for example, cartons for the so-called "fast food" items such as hamburgers may also include such locking devices. These locking devices usually include a male member, often shaped like a button or a hook which fits into a female locking aperture or slot in the other part of the carton. Originally, the locking slots or apertures were formed by punching them out of the pre-formed carton. Not only was this an expensive procedure, requiring separate punching equipment but problems arose from its use, mainly because accurate registering of the pre-formed carton in the punching equipment was required but could not be consistently achieved in practice.
U.S. Pat. Nos. 3,845,187 and 3,862,817 Dahlberg describe a method by which the apertures may be formed in the carton at the same time as the carton is formed. In the method described in these patents, an attempt is made to form the aperture by tearing and scraping the plastic during the moulding cycle. To do this, the moulding dies are fitted with a resiliently mounted former which presses through the plastic sheet in the mould and then scrapes the softened plastic away from the area where the aperture is to be. Although this approach had the advantage of eliminating the need for separate punching equipment, the tearing process produced results which were unpredictable and, in addition, had a tendency to produce apertures which, having scraped or torn edges, were unslightly.
Improved methods for forming the apertures are described in U.S. Pat. Nos. 4,108,941 Kermoian, 4,142,848-Irwin and 4,143,111 Irwin. In the methods described in these patents, shear members in the male and female moulding dies shear the plastic during the closing of the mould to form the holes in the plastic.
In commonly owned U.S. patent application Ser. No. 531,186, filed Sept. 12, 1983 and now U.S. Pat. No. 4,463,894 by J. L. Daines, another improved method of forming the locking apertures is described, employing a biased shearing key in the mould to shear the softened plastic during the moulding process.
In all these methods, however, the apertures which have been produced are essentially planar apertures, that is, apertures which, except for their extent through the plastic sheet, are purely two dimensional. Certain types of carton, however, require a three-dimensional type of aperture in order to achieve the desired locking effect; for example, refer to U.S. Pat. Nos. 3,735,917 Warburton and 3,908,891 Jackson which show locking apertures which are formed into the side walls as well as the back walls of the locking flutes or recesses. None of the shearing mechanisms shown in these earlier patents is capable of forming such an aperture.